Leakage loss accounts for the energy escaping towards the solid silica region surrounding the PCF structure. To quantify its contribution, we included a perfectly matched layer (PML) [1]}, [2]} wrapping the photonic crystal, as shown in Fig. REF , and we calculated the energy flowing on it. To implement a PML in COMSOL®, the complex coordinate stretching function used is \(f(\rho )=\lambda _\text{t}s\rho ^P(1-i)\) [2]}, where \(\lambda _\text{t}\)  is the typical wavelength of the transverse component of the mechanical wave deformation (\({u}\) ), \(s\)  is the scaling factor that tunes the wave amplitude arriving at the edge of the PML, and \(P\)  is the scaling curvature parameter that controls how strong is the amplitude decay inside the PML. The PML parameters used were: \(s=0.25\)  and \(P=2.0\) . We employed \(\lambda _\text{t}=2\pi [(\Omega _\text{m}^{\prime }/V_\text{s})^2-\beta _\text{m}^2]^{-1/2}\), with \(V_\text{s}=3766\)  m/s as the shear bulk velocity in silica, and \(\beta _\text{m}=2\pi (n_\text{eff,p}-n_\text{eff,S})/\lambda =1.86\times 10^5\)  1/m, from momentum conservation. The effective indexes \(n_\text{eff,p}=1.413067\)  and \(n_\text{eff,S}=1.367107\)  are the calculated at \(\lambda =1.55\)  µm for the pump (HE\(_{11x}\) ) and Stokes (HE\(_{21a}\) ), respectively. Finally, \(\Omega _\text{m}^{\prime }\)  is a mechanical frequency initial guess near the eigenfrequencies being sought for.
